For me, Schopenhauer, Bentham, Hume and Singer, hands down, all because of their clear consistent views on ethics, and their rational and / or empiricism.
There are of course others I like very much (Nitchsze stands out) , but the ones above are my favorites.
I really don't like Descartes. He's clearly more intelligent than I am, this is obvious from his pensées, but I can't see how he could justify some of his claims at all (some of them which were very cruel IMO)
For me, Ludwig Wittgenstein is the greatest since Plato.
When I was young, a few decades ago, Aristotle was the rock upon which science was based. However, since then, what with all we've learned about the structure of reality (that physical reality, despite appearances, is mostly empty space; that 75%-90% of the mass in the universe is invisible black matter; that ultimately reality consists of vibrating strings; that there are probably 11 dimensions; that the Big Bang was probably the result of two "branes" colliding.
Now, Plato, for whom the reality of the senses was just a shadow of reality itself, appears to have been closer to the truth and foundation of contemporary physics in many ways.
My favorite contemporary philosopher is Peter Singer.
Hence I like Bentham and Miller, but not everything they said. That's true for every philosopher. I like Kant, but also not everything he said. I think the categorical imperative is a good idea.
If you count Buddhism as a philosophy, which I do, I would add Buddha as well.
Hands down, my favorite philospoher is Spock.
Renee Descarte walks in to a bar. The bartender asks, "Hey buddy, you want a drink?" he replies, "I think not" and *poof* he disappears.
Not to discredit its value but philosophers are just bullshit artists. I think philosophy is just a way to argue something without any facts. I think that it served its purpose when it stimulated thought and inspired the right questions that gave birth to science but philosophy is just an old grandpa with Alzheimer's trying to get his CNA to wash his junk a little more thoroughly. They had their time and in it they were forward thinkers but at the same time they had no clue what they were talking about. I will take a Hypothesis from a scientist over a philosophy from a philosopher any day.